perf/imx_ddr: Assign parents for event_source devices

perf/imx_ddr: Assign parents for event_source devices

Currently all this device appear directly under /sys/devices/
Only root busses should appear there, so instead assign the pmu->dev
parent to be the platform device.

Link: https://lore.kernel.org/linux-cxl/ZCLI9A40PJsyqAmq@kroah.com/
Cc: Frank Li <Frank.li@nxp.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240412161057.14099-16-Jonathan.Cameron@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>